The invention relates to a method for protecting against EEPROM-directed intrusion into a mobile communication device that carries EEPROM memory means and processor means that are interconnected via an electrical interface. A foremost, although not limitative embodiment of such a mobile communication device is a portable telephone. Billing by the telephone authority of a particular person for the costs of the communication is often based on an identifier that is stored within the device in an EEPROM module. This usage of an EEPROM allows the personalizing of the device immediately before issuing it to a holder. Such devices are rendered further unique through storing additional information, such as an Electronic Serial Number. Other such communication devices could be used for acquiring a particular service from a service rendering authority, such as a data bank. In consequence, fraudulent persons have found it advantageous to copy such identifier, and possibly other such informations from the EEPROM of a particular communication device into an other communication device, generally one with identical hardware. In particular, the costs to a user are in particular due to the service rendered, rather than to the hardware. Now, the copying of the EEPROM might be rendered unfeasible by having the EEPROM outputting informations only after an appropriate encryption. However, the present invention recognizes that such devices must by necessity be very inexpensive, and therefore the manufacturing process will produce large series of identical devices that could have various different identifiers and ESN's, but always would have identical encryption mechanisms. This means that copying all information would still lead to an apparently lawful device. The storing of a unique key in the EEPROM itself would be insecure, inasmuch as it would be possible for a fraudulent person to overwrite this key with new, and known information.